1. Field of the Invention
This invention relates to a laser diode array device capable of controlling the optical output at a low power consumption rate and, more particularly, it relates to a laser diode array device to be suitably used for large capacity optical signal transmission applications such as an optical parallel transmission system.
2. Prior Art
In computer system networks comprising a number of computers connected by optical transmission lines, there has arisen a demand for high quality transmission lines for connecting electronic apparatuses and devices that can meet the requirements of, among others, eliminating troubles such as induction and cross talk, allowing a high density assembly of components and having long distance optical signal transmission capabilities.
The technique of optical serial signal transmission for multiplexed signal transmission and that of optical parallel transmission capable of transmitting signals at an enhanced rate are two remarkable improvements achieved in recent years in the technology of optical signal transmission, of which the latter is attracting attention as a promising technique that can be adapted for large capacity computers.
An optical parallel signal transmission system comprises transmission routes, each of which typically has a plurality of optical fibers that are used as optical transmission lines. A light emitting device and a light receiving device are disposed at each of the opposite terminals of the optical fibers for "electric-optical" and "optical-electric" switching.
An array device is preferably used for such a light emitting device because it is a high density device that accommodates a plurality of elements (4 to 50 channels) within a single chip.
More specifically, a laser diode array device is most preferably used as a light emitting device because of its high capacity for optical signal transmission.
It is known that existing laser diode array devices can undesirably change their optical output level in operation as the threshold current level, light emitting efficiency and other characteristics are often affected by ambient temperature and other environmental factors while emitting light.
In order to avoid fluctuations in the optical output level, there have been proposed devices that are externally provided with a feedback circuit for automatic power control (APC), automatic temperature control (ATC) and/or other additional functional features.
Known laser diode array devices are also accompanied by a problem of a relatively high power consumption rate and therefore there has been a demand for laser diode array device having a low threshold current level and an improved light emitting efficiency.
The above identified demand can be met to a certain extent by reducing the cavity of a laser diode array device and coating it with a dielectric film (high reflection film) having a reflectivity equal to or greater than 80%. With such arrangements, laser emitted from the light emitting terminal surface of the device can be oscillated with an improved efficiency.
Proposed means for feeding back the optical output of a laser diode array device include the following.
Current injection method--This is a method of picking up part of light being emitted by a laser diode array device and monitoring the optical output of the device by a photo-detector so that any fluctuations in the level of optical output of the device can be compensated by injecting an electric current into the device.
Separation method--This is a method of dividing part of the stripe-shaped oscillator of a laser diode array device into two section by etching; one for oscillation in the direction of the stripe and the other for oscillation in a direction perpendicular to that of the stripe so that the former may be used as an oscillating section and the other as a light reception monitor section for observing the optical output of the device.